Conventional systems may include storage configurations with two Redundant Array of Independent Disks (RAID) devices, which may be referred to as RAID devices Level 1. Disk mirroring refers to a technique in which data is written to two duplicate disks simultaneously so that if one of the disk drives controlling one of the disks fails, another disk drive controlling the other disk may access a copy of the data. Thus, one of the RAID devices mirrors (i.e., stores copies of) the data of the other RAID device.
A software interface to a disk controller will tell the disk controller which track and sector to read from. To perform the read, the disk controller has to move a read/write head in or out to an appropriate track. The time that it takes to do this is called the seek time. Once the head is correctly positioned, the controller has to wait for the desired sector to spin around so it is under the head. This time is called the rotational delay.
Since the disk drive may receive many requests for reads and writes, the requests are queued. Then, there are several strategies for scheduling I/O events on a disk. For example, one strategy may be referred to as “Shortest Seek Time First”, in which case when a new request arrives, seek distance of the new request from the current request being processed is calculated and the new request is placed into the queue accordingly. Then, the next closest access is serviced when the current request has been serviced. Another strategy may be referred to as “first come first served”, in which case requests are processed in the order that they are received. Yet another strategy may be referred to as an “elevator algorithm,” in which case requests come in to go to a particular track on a more or less random basis, and the read/write head goes in one direction as long as there are requests in that direction, and then goes in the other direction as long as there are requests in the other direction. At any given instant, the read/write head is either moving in toward the center of the disk or out toward the outside. If the read/write head can satisfy a new request by moving in the same direction, the read/write head does so, but if the read/write head has to switch directions and there are additional requests that could be satisfied without changing direction, the read/write head will not satisfy the new request until the read/write head turns around.
In order to locate data today, conventional systems implement a strategy for scheduling an I/O event on a disk on one of the RAID devices, while ignoring the other one of the RAID devices.